The present invention relates to a nondispersion two-beam, infrared gas analyzer with negative filtering.
The VDI-Report No. 97 of 1966 discloses on pages 17 and 18 a two infrared beam gas analyzer with positive and negative filtering. The two beam paths each include a radiation receiver-detector, a cuvette or cell for test gas (carrier gas plus gas to be detected), and an additional cell. For negative filtering that additional cell is filled e.g. with gas of the same type to be detected. This additional cell can also be called a selectivity chamber. Gas of the same type to be detected is additionally used in the detector.
With regard to negative filtering, the concentration of the gas to be detected in the detection chamber must be sufficiently high, so that a significant portion of the infrared energy, particularly within the particular absorbtion band(s) of the gas to be detected is, in fact, being absorbed. As outlined in this paper, the so-called transverse or cross-sensitivity of the analyzer can be eliminated in that fashion.
The cross or transverse sensitivity introduces an error into the measuring result and arises particularly if the test gas includes components which have absorbtion bands overlapping any of the bands of the gas to be detected. The method of negative filtering is clearly preferred over devices operating with positive filtering, because the afore-mentioned kind of interference can be significantly offset in this manner. Concerning particulars in the known selectivity cell, it contains not only the same type of gas as the one to be detected, but an additional filler or carrier gas, e.g., nitrogen is also included. The filler or carrier gas chosen for that purpose must not have absorption bands, overlapping with those of the gas to be detected; and this is true generally for nitrogen. The pressure in the selectivity cell is usually atmospheric.
Negative filtering when carried out in this manner does, indeed, eliminate cross-sensitivity as produced by other gases which happen to be in the test gas. However, negative filtering as practiced thus far reduces sensitivity in the detection. True trace analysis is no longer possible.